Electronic devices often employ light emitting diodes (LEDs) to indicate the activity or inactivity of the devices. In order to operate within specified parameters, LEDs typically require a relatively narrow range of direct current and voltage. As a result, to use an LED as status indicator, it is customary practice to employ a series, current-limiting resistor to adjust the voltage provided to the LED which, in turn, controls the current through, and the brightness of, the LED for a given application.
In certain devices, such as in data communications devices, a general purpose voltage source can be used to drive an LED. For example, as illustrated in the schematic of Prior Art FIG. 1, a device 10 includes an integrated circuit (IC) (e.g., physical layer (PHY)) 12, having a driver 13 and power supply pins 14, 16 where the power supply pin 14 couples to a positive supply rail 18 and the power supply pin 16 couples to a negative supply rail 20 (e.g., ground). The device 10 also includes an LED 22, and a current limiting resistor 24 coupled between the supply rail 18 and IC 12. In use, the driver 13 causes the output voltage VOUT to equal to the supply voltage VDD (e.g., the driver 13 pulls the supply voltage VOUT to VDD) thereby causing a current to flow through, and activate, the LED 22. The amount of current that flows through the LED 14 is related to the supply voltage (VDD), the output voltage (VOUT), the LED voltage drop (VLED) and the resistor value (R) and is governed by the equation:ILED=(VDD−VOUT−VLED)/R. The brightness of (e.g., the amount of light emitted by) the LED 22 is proportional to the amount of current running through the LED 22.
When the supply voltage VDD is relatively large, the current that flows through the LED 22 is substantially constant. For example, in the case where the supply voltage VDD is 5V, the current that passes through the LED 22 can be between about 11 mA and 9 mA, resulting in a current tolerance between +/−11%. As a result, the brightness of the LED 22 is substantially constant over time.